Years after the Exxon Valdez Oil Spill of 1989, Prince William Sound, Alaska continues to be adversely affected. The severity of the damage from a spill and how long the damage lasts is influenced primarily by the chemical changes the oil undergoes during and after the spill. The process by which the spilled oil changes both physically and chemically is referred to as oil weathering and involves a series of sub-processes including spreading and drift, evaporation, natural dispersion, emulsification and biodegradation. These processes are influenced in turn, by factors including composition of the spilled oil, the duration time of weathering, the type of oil being weathered, temperature, and wave action. For example, lighter oils will generally weather differently than heavier oils. However, some light oils contain waxes that make it behave more like a heavy oil resulting in longer weathering processes. Also, oils with a lower specific gravity (lighter oils) will be less persistent than oils with a higher specific gravity (heavier oils).

Oil weathering sub-processes

Spreading and drift

Spreading and drifting of the oil occurs immediately after oil mixes with water and occurs as a result of interacting forces including gravity, wind and surface tension. This process is also directly influenced by the viscosity or fluidity of the oil. The less viscous the oil, the more it will spread and increase the area of water covered by an oil slick. Additionally, underwater tides and currents, the weather and many other factors influence the rate of spreading. Spills have been known to spread hundreds of kilometers in a matter of days due in part to high currents.

Evaporation

Evaporation involves the loss of lower molecular weight, more volatile molecules and can be responsible for the loss of 20 to 50 percent of the spilled oil.[1] For these oils, spreading increases evaporation rates because it increases the surface area of the spill allowing for more evaporation. Additionally rough seas; high wind speeds and warm temperatures can increase evaporation rates.

Natural dispersion

Natural dispersion is the process by which small droplets of oil are formed and integrated into the water column by the energy of breaking waves. These oil droplets may either remain in suspension if they are very small in size, or float back to the water surface if they are fairly large. The large droplets that float back to the water surface may coalesce and form an “oil sheen”. Particularly viscous oils disperse more slowly and can persist in the ocean for prolonged periods of time. Dispersion of the oil results in lowering the localized concentration of the oil.

Emulsification

Wave action and wind cause emulsification -- the incorporation of water droplets into the oil. Emulsification changes the viscosity and the thickness of the oil slick and can make the clean up more difficult. Some oils are easier to emulsify than others and one factor that affects emulsification rate is the metal content of the oil. For example, oils with high asphaltene content will promote emulsification. Asphaltenes, complex compounds containing metals such as nickel and vanadium, are the heaviest and most aromatic components found in crude oil. More viscous oils will emulsify at slower rates than less viscous oils. Emulsions become more viscous and stable as more water is incorporated into the oil (70%-80% water).[2] Once the oil is emulsified, it remains in that state and can stay on the sea surface for prolonged periods of time. In some cases, the oil has to be weathered by other processes (such as evaporation) before emulsification. Unlike dispersion or evaporation which reduce the amount or concentration of oil, emulsification is increases the volume of oil, sometimes by as much as four-fold.[2]

Biodegradation

When marine microorganisms metabolize oil compounds, it is referred to as biodegradation. As with all of the weathering sub-processes, biodegradation is influenced by oil properties, temperature and, in addition, availability of oxygen and nutrients. Because different microorganisms degrade specific hydrocarbons, multiple species of microorganisms must be involved for biodegradation to occur. Populations of specific microorganisms increase during a spill following the increased availability of nutrients. However, as nutrients and oxygen once again become limiting, populations return to their natural state. Larger and more complex molecules are more difficult to biodegrade.

Because the sub-processes of natural dispersion and emulsification increase the available surface area, they encourage biodegradation. As a result, biodegradation is often the final step in oil weathering and also often responsible for eliminating the last traces of oil.

Examples

Amoco Cadiz spill

The Amoco Cadiz oil spill occurred in 1978 off the coast of France spilling 68 million gallons of oil.

The spill quickly began to emulsify, creating what scientists called a “mousse”, while an estimated one third of the oil was evaporated.

The remaining oil was absorbed into denser material and pulled vertically down the water column and this material became imbedded in the surrounding beaches and salt marshes.

Once imbedded in sediments, the oil was biodegraded by microbial action.

Exxon Valdez spill

The Exxon Valdez spill of 1989 spilled more than 10.5 million gallons of oil into the Prince William Sound of Alaska, USA. The Prince William Sound (PWS) is an area that experiences “extreme storms, waves and tidal action”. These natural actions were instrumental in weathering the spilled oil. Only a few small patches of oil slick remain in PWS and these patches are found mainly in areas with moderate to low wave energy. These areas were also noted to have larger boulders and cobbles. The remaining oil from the spill has mostly been covered by new sediment is slowly leaching into the surrounding environment.

This article was researched and written by students at Mount Holyoke College participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed and approved by EOE editors, and in many cases individual experts in the appropriate fields.